7382 Sogah, and D. J. Cram, ibM., 95, 2691 (1973): (e) D. J. Cram and J. M. Cram, Science, 103,803 (1974). (a) D. J. Cram and J. M. Cram, Accounts of Chem. Res., 4, 204 (1971); (b) D. J. Cram, R. E. Hornby, E. A. Truesdale, H. J. Reich, M. H. Delton, and J. M.Cram, Tetrahedron, 30, 1757 (1974). H. J. Reich and D. J. Cram, J. Amer. Chem. SOC.,91,3527 (1969). (a) Compounds gave carbon and hydrogen analyses within 0.30% of theory; (b) the pmr spectra in CDCb were consistent with their assigned structures; (c) the mass spectrum (70 eV) contained the molecular ion. D. L. Fields, J. B. Miller, and D. D. Reynolds, J. Org. Chem., 29, 2640 (1964). (a) This new procedure for making [2,2]paracyclophanes was invented by Roger C. Heigeson for another purpose (University of Wisconsin at Milwaukee, 1970). (b) This compound was previously prepared and characterized (ref 7a). W. Rebafka and H. A. Staab (Angew. Chem., Int. Ed. Engl., 12, 776 (1973); 13, 203 (1974)) prepared 4,7,12,15- and 4,7,13,16-tetrahydroxy[2.2]paracyclophane by demethylation of their corresponding ethers but dd not characterize them. H. E. Winberg, F. S. Fawcett, W. E. Mochel, and C. W. Theobald. J. Amer. Chem. Soc.. 82, 1428 (1960). J. M. Timko, R. C. Helgeson, M. Newcomb, G. W. Gokel, and D. J. Cram, J. Amer. Chem. Soc.. 96, 7097 (1974). H. J. Reich and D. J. Cram, J. Amer. Chem. SOC., 91, 3517 (1969).
Roger C. Helgeson, Joseph M. Timko, Donald J. Cram* Contribution No. 3357, Department of Chemistry University of California at Los Angeles Los Angeles, California 90024 Received July 22, 1974
An Electron Spin Resonance Study of the Stereomutation in 1,1,3,3-TetrafluoroallylicRadicals Sir: It is well known that simple allylic radicals are remarkably stable toward geometrical isomerization.' In a recent high-temperature esr study, we have shown that the allyl radical is static on the esr time scale even at 280' precluding a AG* for internal rotation of less than 17 kcal/moL2 We wish to report now that the replacement of the terminal protons in the allyl radical by fluorines markedly increases the rate of internal rotation about the allylic bonds.3 We find, furthermore, that halogen substituents in the 2 position (Ib, c) further decrease the activation energy to internal rotation (C1 > F). The latter effect is a manifestation of radical stabilization in P-haloalkyl radicals which is a topic of lively current interest4 X
Ia, X = H b,X=F c, X = c1
- 102' with the evolution of a triplet-of-triplets spectrum appropriate for two pairs of inequivalent fluorines. A fitting of the spectra by density-matrix line shapes calculations7 based on noncorrelated exchange of the two pairs of syn and anti fluorines indicates a rate of 3 X lo7 sec-' at -80' and a AG* of 4.5 kcal/mol for the exchange process. The esr spectra of Ib (Figure 1) and Ia (Table I) can be obtained by the same method and are similarly temperature dependent. 'Fluxional spectral behavior, however, occurs at higher temperatures. A line shape analysis leads to Arrhenius plots from which activation free energies of 6.1 and 7.2 kcal/mol, respectively, can be extracted. A mechanism' for stereomutation based on a concerted exchange of the terminal fluorines brought about by disrotatory or conrotatory motion of the -CF2 termini concomitantly with the closing of the CIC2C3 angle cannot be excluded a priori. Calculated spectra for this mechanism do show a different evolution of the line shapes but the differences can easily be confused with the effects of dipolar broadening. The observation of the isomeric 2,2,3,3-tetrafluorocyclopropyl radicals from the corresponding bromides,8 however, militates against such mechanism. INDO calculations9 for the coplanar perfluoroallyl (Table 11) predict remarkably well the isotropic coupling constants for the terminal fluorines. The agreement is less satisfactory for F2 although the negative sign for this coupling is in fact borne out by the experimental temperature dependence of its magnitude (Table I). Since the vibronic contribution to this coupling should become more positive as the temperature increases, owing to larger amplitudes for out-of-plane motions and hence increased possibilities for direct spin delocalization, the observed decreasing magnitude of a(F2) with increasing temperature clearly demands a negative sign for this coupling in the ground vibronic state. The calculations also predict a substantial unpaired spin density in the pz orbitals on the terminal fluorines (1 9%) indicating an extension of the r allylic MO system over these fluorines. This result is consistent with the large anistropy of the A tensor inferred above from the pronounced dipolar line broadening effects. The dramatic lowering of the rotational barriers in these radicals relative to the allyl radical itself is not entirely un-
X
I1
The 2-chlorotetrafluoroallyl radical IC was generated photochemically in a cyclopropane-ethane solution from 2chloro-3-bromotetrafluoropropene by reaction with tri(nbuty1)silyl radicals as bromine abstracting agent^.^ At -52' the esr spectrum consists of a quintet of 43.3 G without resolvable C1 hyperfine structure indicating four equivalent fluorines on the esr time scale. Lowering the temperature produces two types of line shape effects: first, a strongly viscosity-dependent broadening of all lines except the central peak and, second, an additional broadening of the M I = f l and MI = 0 lines. The first effect is related to the modulation of the large anisotropic component of the fluorine hyperfine A tensor by molecular tumbling (dipolar broadening)6 while the second is caused by a slowing of the exchange of the terminal fluorines. The latter is complete at Journal of the American Chemical Society
/
96:23
/
t
,
'
I
Figure 1. Esr spectra of the perfluoroallyl radical in cyclopropane. The calculated spectra (density matrix including higher order effects) refer to three conditions for the nonconcerted exchange of the terminal fluorines: fast exchange (top), k = 2.5 X lo7 sec-' (middle), and slow exchange (bottom).
November 13, 1974
7383 Table I. Esr Spectral Parameters of Fluoroallylic Radicals" "Ranti
R
R = H R = H R = H R1.i = F, R3,3 = H R = F
-----Magnitudes Temp, "C
Rvn
X = Hc X=H' X = F X = W
- 130
(d
of the H and F isotropic hyperfine coupling constants (G)-----a(Rd a(Rsyn) a(R),", a(X) AG*,b kcal/mol
2. 00254d
14.81 14.82 14.86 F: 34.50 H: 15.98 42.56
14.36 4.06 >17" 14.41 4.53 14.37 9.18 2.00325 F: 31.90 H : 15.60 3.50 X = H -46 2.00424 41.14 3.33 7.2 $26 41.62 3.60 R = F X = F -95 2.00426 46.35 40.07 43.21 6.27 6.1 - 54 47.00 40.59 43.80 4.96 +40 43.97 3.97 R = F x = c1 - 102 2.00382 44.89 40.71 42.80 f 4.5 - 52 43.30 f a The assignments of the larger coupling to the anti substituent on each terminal carbon atom are based on internal consistency with the 1-methallyl radicals (cf. ref IC). Activation free energy for the exchange of the terminal substituents. c Reference IC. R. H. Fessenden and R. W. Schuler, J. Chem. Phys., 39,2147 (1963). e Reference 2. f Unresolved. +230 - 60 - 82
13.90 14.00 13.88 F:29.29 H: 15.21 39.72
Table 11. Selected INDO Parameters for Allyl and Perfluoroallyl Radicals
Radicak
PdC1,3)
Unpaired p n Spin densities Ps(C*) B z (F1,3)*
CH~=CHCH*~
0.6240
-0.2419
CF~CFCF~~
0.5453
-0.2628
0,0482
---H and F isotropic hyperfine--coupling constants (G)c PS
(Fz)
-0.0207 ~
1,3b
2
-14.54 (-)14,36(exptl) 39 40 (+)43.21(exptl)
6.51 (+)4.06(exptl) -19 83 (-Ti.27(exptl)f
a Trigonal planar geometry energy optimized by varying the C-C distances. * Average values for syn and anti substituents. E3perimental signsFonsistent with the signs of the temperature coefficients (cf. text). d(CH) = 1.08 A, d(CC) = 1.38 A. e d(CF) = 1.33 A, d(CC) = 1.39 A. f Unusually temperature-dependent increasing in magnitude with decreasing temperature (cf text).
derstood as yet. Stabilization of the transition state I1 by the a-fluorine substituents seems unlikely since, on the basis of C-H bond dissociation energies, difluoromethyl and methyl radicals have comparable stabilities.1° Steric repulsion between the syn fluorines in the planar structure I no doubt contributes to the destabilization of the ground state. We believe, however, that the effect is primarily of electronic origin and may be associated with a ground state destabilization due to electron-electron interaction. As indicated by the INDO calculations, the A MO's of Ib are delocalized over all atoms. By perturbation theory, the interaction of the PA fluorine lone pairs with the singly occupied allyl A MO is weakly stabilizing while that with the doubly occupied A MO is destabilizing.]' The magnitude of these conflicting effects should be inversely proportional to the energy separation between the interacting orbitals. The very low energy of the fluorine lone pairs12 requires that the smaller energy separation and hence the larger interaction occurs between the lone pairs and the doubly occupied allyl A MO. This destabilizing interaction should therefore dominate, reducing the net allyl resonance energy and consequently lowering the rotational barrier. The trend displayed by AG* upon halogen substitution in the 2 position is best interpreted in terms of a stabilization of the perpendicular transition state I1 which is analogous to a 6-chloroalkyl radical. These radicals are known to prefer the conformation in which the C1 atom is eclipsed by the half-filled p orbital, and a twofold barrier of about 4 kcal/mol has been estimated for CH2CH2C1.4C The CH2CH2F radical also has some proclivity to adopt the same conformation although in. this case the latter represents only one of two stable r ~ t a m e r s The . ~ ~ details of this study, together with similar data for hydrocarbon allylic radicals substituted in the 2 position by heteroatoms, will be presented elsewhere.
References and Notes (1)(a) R. J. Crawford, J. Hamelin, and B. Strehlke, J. Amer. Chem. Soc.. 93, 3810 (1971):(b) C. Walling and W. Thaler, ibid., 83, 3877 (1961):(c) J. K. Kochi and P. J. Krusic, ibid., 90, 7157 (1968). (2)P. J. Krusic, P. Meakin. and 6. E. Smart.ibid., 96, 621 1 (1974). (3) Recently the l,ldicyano-2-tert-butylallyl radical has been found to undergo relatively fast rotation about the c 1 - C ~allylic bond. Line-shape analysis yielded a rotational activation energy of 9.0 kcallmol. (R. Sustmann and H. Trill, J. Amer. Chem. SOC., 96, 4343 (1974)). (4)(a) P. S. Skell, "Free Radicals," Wiley-Interscience, New York, N.Y., 1973,Chapter 26;(b) L. Kaplan, "Bridged Free Radicals," Marcel Dekker, New York, N.Y., 1972;(c) K. S.Chen, 1. H. Elson, and J. K. Kochi, J. Amer. Chem. SOC., 95, 5341 (1973);(d) I. H. Elson, K. S. Chem, and J. K. Kochi, Chem. Phys. Lett., 21, 72 (1973):(e) D. J. Edge and J. K. Kochi, J. Amer. Chem. SOC., 94, 6485 (1972);(f) M. C. R. Symons, Chem. Phys. Lett., 19,61 (1973);(g) A. J. Bowles, A. Hudson, and R. A. Jackson, ibid., 5, 552 (1970);(f) K. S.Chen. P.J. Krusic, P. Meakln, and J. K. Kochi, J. Phys. Chem.. 78, 2014 (1974). (5) A. Hudson and R. A. Jackson, Chem. Commun., 1323 (1969). (6) (a) J. Cooper, A. Hudson, R. A. Jackson, and M. Townson, Mol. Phys., 23, 1155 (1972):(b) P. Meakin and P. J. Krusic, J. Amer. Chem. SOC., 95, 8185 (1973). (7)P. J. Krusic, P. Meakin, and J. P. Jesson, J. Phys. Chem., 75, 3438
(1971). (8) B. E. Smart and P. J. Krusic, manuscript in preparation. (9)J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory," McGraw-Hill, New York. N.Y., 1970.We obtained a copy of the program CNINDO, written by P. A. Dobosh, from the Quantum Chemical Program Exchange, Indiana University. (10)K. W. Egger and A. T. Cocks, Helv. Chim. Acta, 56, 1516,1537 (1973). (11) R. C. Bingham and M. J. S. Dewar, J. Amer. Chem. SOC., 95, 7180 ( 1973). (12)D. W. Turner, C. Baker, A. Baker, and C. R. Brundle, "Molecular Photoelectron Spectroscopy," Wlley-lnterscience, New York, N.Y., 1970. (13) Organic Chemicals Department.
B. E. Smart, P. J. Krusic* P. Meakin, R. C. BinghamI3 Contribution No. 2181 Central Research Department E. I . du Pont de Nemours & Company Wilmington, Delaware 19898 Received June 19. 1974 Communications to the Editor
